As is known in the art, there is a need for radio-frequency (RF) power amplifiers and inverters that can provide high efficiency across a wide range of output powers. In some cases, it may be desirable to simply be able to generate ac output power efficiently at multiple different output power levels or to maintain a narrow range of ac output amplitude as the dc input varies over a wide range. Such approaches may be taken, for example, in applications relating to some kinds of RF transmitters, as well as inverters for dc-dc converters.
In other cases, it may be desirable to achieve high efficiency as output power is varied continuously over a wide range, while at the same time meeting strict linearity and dynamic requirements. Applications falling into this category include audio power amplifiers and RF power amplifiers for magnetic resonance imaging (MRI) and for RF communications, including cell phone base stations and handsets.
One approach for building an inverter or power amplifier system that maintains high efficiency over a wide output power range is to provide a means for the inverter or power amplifier supply voltage to be switched among multiple discrete voltage levels. In this approach, lower voltage levels are provided to the power amplifier when low output power is desired, and higher voltage levels are provided to the power amplifier when high output power is needed. Instantaneous output power can be controlled to finer resolution by a secondary means of control, such as using a linear regulator or other added converter to further control bias voltage. Such techniques are taught, for example, in: U.S. Pat. No. 7,482,869, issued Jan. 27, 2009 entitled “High Efficiency Amplification;” as well as in F. H. Raab, “Average Efficiency of Class-G Power Amplifiers,” IEEE Transactions on Consumer Electronics, Vol. CE-32, no. 2, pp. 145-150, May 1986; and J. S. Walling, S. S. Taylor, and D. J. Allstot, “A Class-G Supply Modulator and Class-E PA in 130 nm CMOS,” IEEE Journal of Solid-State Circuits, Vol. 44, No. 9, pp. 2339-2347, September 2009 (which describes a version of the so-called “Class G” technique).
The approach of switching among multiple discrete voltage levels may also be implemented by providing delta sigma modulation or other discrete modulation among the power supply levels, providing pulse-width modulation, pulse density modulation, frequency modulation, drive amplitude modulation of the power amplifier, or phase-shift control or outphasing of two or more power amplifiers. Systems of this latter type include multilevel LINC (MLINC) Power Amplifiers Y.-J. Chen, K.-Y. Jheng, A.-Y. Wu, H.-W. Tsao, and P. Tseng, “Multilevel LINC Transmitter,” U.S. Patent Application Publication 2008/0019459 A1, Jan. 24, 2008 and J. Hur, K.-W. Kim, K. Lim, C.-H. Lee, H. Kim, and J. Laskar, “Systems and Methods for a Level-Shifting High-Efficiency LINC Amplifier using Dynamic Power Supply,” U.S. Patent Application Publication 2010/10073084 A1, Mar. 25, 2010 and Asymmetric Multilevel Outphasing (AMO) Power Amplifiers J. L. Dawson, D. J. Perreault, S. Chung, P. Godoy, and E. Huang, “Asymmetric Multilevel Outphasing Architecture for RF Amplifiers,” U.S. Patent Application Publication, US 2010/10117727 A1, May 13, 2010 (which application is assigned to the assignee of the present invention).
In these systems, it is desirable to synthesize the multiple levels used by the inverters/power amplifier(s) or used by the voltage regulation circuitry that provides the continuously variable supply voltage to the power amplifier(s) as efficiently and compactly as possible. One approach for doing so is to synthesize multiple static dc levels simultaneously, and switch among them by a switching network connected between the dc outputs and the power amplifier or between the dc outputs and further regulation circuitry. This generation of multiple levels can be accomplished using a multiple output magnetic dc-dc converter (such as a multioutput flyback converter) operated from a single supply, or through the use of a plurality of dc-dc converters, as Illustrated in M. Vasic, O. Garcia, J. A. Oliver, P. Alou, D. Diaz, J. A. Cobos, “Comparison of Two Multilevel Architectures for Envelope Amplifier”, 2009 Industrial Electronics Conference, pp. 283-289, Nov. 3-5, 2009.